The present invention is generally directed to a method and apparatus for monitoring the condition of electric motors. More particularly, it relates to a monitor system integrated into an electric motor to monitor and record various operating parameters during the life of the motor for the purpose of predicting failures and evaluating motor life.
Electric motors, particularly alternating-current (AC) induction motors, are heavily employed in industrial and manufacturing facilities. AC induction motors are used to provide mechanical inputs for machinery in manufacturing facilities. Manufacturing downtime caused by a failure of an electric motor reduces productivity and profitability. Electric motors, therefore, are important elements of industrial facilities, and the health and condition of these motors must be closely monitored to anticipate and prevent motor failures that could result in costly unscheduled downtime.
Evaluating the extent of stator winding insulation degradation is a primary concern in determining the condition of an electric motor. As the motor ages, insulation breakdown occurs due to high motor temperature and other operating stresses. When insulation degradation reaches a critical point, the motor windings short together. Eventually, these faults short to ground, resulting in catastrophic motor failure.
Previous attempts have been made to provide electric motor monitoring systems capable of monitoring the condition of the motor. Many of these systems focus on measuring motor temperature, and estimating, based on the measured temperature, the degree of insulation degradation. Such systems use this estimation of insulation degradation as a way of predicting the remaining useful life of the motor.
While high motor temperature, and resultant insulation degradation can contribute to electric motor failures, it is not the only factor. There are many complex and interrelated operating characteristics of electric motors that affect the health and longevity of the motor. Factors such as loading, vibration, supply voltage quality, environmental conditions, and the number of motor starts/stops affect motor life. Thus, improved and more comprehensive motor monitors are needed.
The present invention addresses this and other needs by monitoring the operational condition of an electric motor, and storing in a memory device general trend, prognostic, diagnostic, and hazardous event information indicating the motor""s operational condition. The invention records characteristics that indicate the operational condition of the motor, such as vibration, temperature, magnetic flux, and the voltages applied to the motor""s windings.
Upon the occurrence of a first circumstance, such when measurements of the motor""s speed, winding temperature, and voltage, indicate that the motor is operating within its normal load profile, prognostic information is extracted from the sensor signals. The prognostic information provides a profile of the motor""s operational condition over time without the influences of fluctuating loads, temperature, and voltage.
Upon the occurrence of a second circumstance, such as when the motor""s load factor is within a load factor range that is most common for the motor, diagnostic information is extracted from the sensor signals. The diagnostic information indicates the motor""s operational condition when it is operating within its most common load zone.
Upon occurrence of a third circumstance, such as an indication that the motor is operating abnormally, hazardous event information is extracted from the sensor signals and stored. The hazardous event information indicates the motor""s condition at the time that a hazardous event, or fault, occurred.
Preferred embodiments of the invention measure and store general trend information at regular intervals to determine short- and long-term operational trends over time. The general trend information provides additional information about the motor""s condition when the motor is not operating within a normal range.
During a first time period, a preferred embodiment of the invention operates in a learning mode. At periodic intervals while in the learning mode, the invention determines values, such as voltage, temperature, and speed, from the sensor signals. The invention determines into which of several predetermined ranges each sensor value falls at each interval. At each periodic interval, a prognosis profile value is determined based on the sensor values. The invention determines a normal range for the prognosis profile value, which is the range within which the prognosis profile value most often falls during the first time period.
During a second time period that follows the first time period, the invention extracts prognostic information from the sensor signals, such as motor vibration, flux, and voltage spectra.
During the second time period, this prognostic information is stored in the memory device at periodic intervals only when the prognosis profile value falls within the normal range as determined in the learning mode.
Thus, by storing the vibration, flux, and voltage spectra only when the motor is operating within the typical range for these characteristics, the invention ensures that the vibration, flux, and voltage spectra that are stored in the memory are truly representative of the typical operational condition of the motor.
In another aspect, the invention provides an apparatus for monitoring operational characteristics of an electric induction motor over a period of time. The apparatus has sensors integrated into the motor for sensing characteristics that indicate the operational condition of the motor. These sensors generate sensor signals related to the sensed characteristics. Based on the sensor signals, a processor determines values, indicative of the motor""s operational condition, at periodic measurement intervals during the period of time. The processor also determines into which of several predetermined value ranges that the values fall at each periodic measurement interval. Based on the number of times that the values fall into each of the predetermined load ranges during the period of time, the processor generates count values corresponding to the individual value ranges. A memory device stores the count values generated by the processor. Collectively, these count values form histograms that are stored in the memory device.